1. Field of the Invention
This invention relates to an intake air heating system of a combined cycle plant, which is designed, particularly, to be operable even in an extremely cold district.
2. Description of the Related Art
In an iron making process, a large amount of BFG (blast furnace gas) is formed in a blast furnace. In recent years, a BFG-fired combined cycle plant, which uses this BFG as a fuel for a gas turbine, has attracted attention from the viewpoints of environmental protection and effective utilization of energy.
BFG is in a special fuel state such that it has a lower calorific value and contains larger amounts of impurities in the fuel, than a natural gas which is usually used as a gas turbine fuel.
In the BFG-fired combined cycle plant, therefore, BFG is used as a fuel, with various ideas including, for example, the idea that BFG is compressed in a gas compressor directly connected to a gas turbine shaft, and then burned in a combustor.
The installation of such a BFG-fired combined cycle plant in an extremely cold district as well as in a warm region is under consideration.
In installing the BFG-fired combined cycle plant in the extremely cold district, the following problems occur, because the calorific value of BFG is low and the air temperature of the atmosphere taken in is very low (e.g., −30° C.)    (1) The ignition performance of the gas turbine deteriorates.
That is, under extremely low atmospheric temperature (e.g., −30° C.) conditions, it becomes difficult to ignite the gas turbine.    (2) Combustion vibrations in a no-load state are apt to occur.
That is, under extremely low atmospheric temperature (e.g., −30° C.) conditions, combustion vibrations tend to occur at an increased turbine speed or in a state close to a no-load state. At the increased turbine speed or in a low-load state, therefore, it becomes difficult to make an adjustment so that combustion vibrations do not occur.    (3) Freezing or icing of moisture in intake air at the inlet of the compressor occurs.
If the temperature of the air taken in is low, moisture in the low temperature air has a possibility to freeze or ice at the inlet of the compressor.
Thus, an anti-icing technology, which bleeds part of air in the casing of the compressor, and incorporates the bleed air into intake air to heat the intake air, has already been developed. With this technology, however, air within the casing of the compressor is bled. Thus, combustibility is adversely affected, and a large amount of air cannot be bled, so that the range of a rise in the intake air temperature has been restricted.
Even if such a technology is applied to the BFG-fired combined cycle plant installed in the extremely cold district, it is impossible to ensure the amount of heating bleed air required for heating air at an extremely low temperature (e.g., −30° C.), because of limitations on the amount of bleed air.    (4) Surging is apt to occur in the air compressor under load in the vicinity of rated load.
That is, under extremely low atmospheric temperature (e.g., −30° C.) conditions, surging is apt to occur under constant load. Thus, the compressor is operated in a limited manner so as not to cause surging. This requires limitations on load, thereby ma king operation under heavy load difficult.